1. Field of the Invention
This invention relates to a biochemical analysis apparatus for measuring the activity of an electrolyte contained in a liquid sample by measuring a difference in potential between the liquid sample and a reference solution.
2. Description of the Prior Art
As disclosed in, for example, Japanese Unexamined Patent Publication Nos. 58(1983)-211648 and 59(1984)-30055 and Japanese Patent Application No. 59(1984)-11744, which corresponds to Japanese Unexamined Patent Publication No. 60(1985) - 155960, there has been proposed an electrolyte determination slide for receiving a liquid sample such as a wine, a beverage, service water, particularly a body fluid (blood, urine, saliva or the like), which is applied by spotting, and quantitatively analyzing the activity or concentration of a predetermined electrolyte contained in the liquid sample by potentiometry.
In general, the electrolyte determination slide is provided with at least one set of electrolyte selective electrode pair having an electrolyte selective layer selectively responding to a predetermined electrolyte as the outermost layer. The electrolyte selective electrode pair is supported between an upper frame and a lower supporting frame. The upper frame is provided with a pair of liquid receiving holes positioned to correspond to the set of the electrolyte selective electrode pair. A porous bridge (which should preferably be constituted by twisted fiber yarn) for achieving electrical conduction, i.e. liquid-junction, between a liquid sample applied to one of the pair of the liquid receiving holes and a reference solution applied to the other thereof is disposed, usually, on the upper frame. In general, in the case where multiple sets of the electrolyte selective electrode pairs are provided, a pair of porous liquid distributing members for communicating the electrolyte selective electrode pairs with the pair of the liquid receiving holes are disposed between the upper frame and the electrolyte selective electrode pairs.
In the case where the electrolyte determination slide having the aforesaid configuration is provided with, for example, three sets of the electrolyte selective electrode pairs responding respectively to Na.sup.+, K.sup.+, and Cl.sup.- electrolytes, a reference solution having known activity values of these electrolytes is applied to one of the pair of the liquid receiving holes, and a liquid sample wherein the activity values of these electrolytes are unknown is applied to the other of the pair of the liquid receiving holes. The reference solution and the liquid sample penetrate through the porous liquid distributing members to the corresponding electrolyte selective electrodes. On the other hand, the reference solution and the liquid sample contact each other to achieve liquid-junction near the center of the porous bridge, and electrical conduction is effected between the reference solution and the liquid sample. As a result, a difference in potential proportional to the difference in activity of each electrolyte between the reference solution and the liquid sample arises between the electrodes of each electrolyte selective electrode pair. When the differences in potential are measured, it is possible to measure the activity values of the Na.sup.+, K.sup.+ and Cl.sup.- electrolytes contained in the liquid sample simultaneously, sequentially or when necessary, based on the measured values and calibration curves determined in advance from the activity values of the electrolytes in the reference solution (by use of the Nernst's equation).
With the aforesaid electrolyte determination slide, it is possible to measure the electrolyte activity simply by applying the liquid sample and the reference solution only once. Therefore, the electrolyte determination slide is very advantageous for analysis of an aqueous liquid sample, particularly for clinical analysis of a sample such as blood taken from the human body.
At the time the liquid sample and the reference solution are applied to the electrolyte determination slide, they should be applied as simultaneously as possible. Therefore, in order to simultaneously apply the liquid sample and the reference solution, a pipette device provided with two sample applying nozzles as disclosed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-173131, which corresponds to U.S. Pat. No. 4,734,261, has heretofore been utilized.
The pipette device provided with two sample applying nozzles simultaneously sucks up and accommodates the liquid sample and the reference solution. On the other hand, the distance between the centers of a pair of the liquid receiving holes of the electrolyte determination slide is very short and is usually not longer than approximately 10 mm. In the case where the pipette device is constituted so that the distance between the centers of the two sample applying nozzles is as short as the distance between the centers of the liquid receiving holes of the electrolyte determination slide, it is not always possible to immerse one of the two sample applying nozzles in the liquid sample and simultaneously immerse the other of the two sample applying nozzles in the reference solution. Therefore, the pipette device is constituted so that the distance between the two sample applying nozzles can be slightly increased at the time of sucking of the liquid sample and the reference solution. Accordingly, the pipette device has a complicated mechanism. Also, in the case where the pipette device is to be automated and used in an apparatus, a more complicated mechanism is necessary. As a result, the cost of the apparatus becomes high, operation failures increase, and reliability of the apparatus is adversely affected.
Also, the apparatus must be constituted so that the liquid sample and the reference solution are accommodated at positions spaced from each other by a short distance equal to the distance between the sample applying nozzles, and the liquid sample and the reference solution are simultaneously taken up from the positions of accommodation. Therefore, the layout of the elements of the apparatus is limited, and cannot always be adjusted to be suitable for making the apparatus small and cheap.
Moreover, a plurality of liquid samples are generally accommodated in small amounts and sequentially used for sample application. On the other hand, the same reference solution can be utilized in common for a plurality of the liquid samples. Therefore, from the viewpoint of the operating efficiency in use of the apparatus, it is desired that the reference solution be accommodated in a large tank or the like so that it need not frequently be replenished.